I pulled the trigger because of a special offer ($120 off at my local dealer) and I purchased this flash unit, intrigued by the advertised unique features of this flash like:
- Guide Number (ISO100): (105mm beam angle) 60, (35mm beam angle) 40
- FULL ~ 1/128: light-emitting sub-flash
- Color LCD (with auto-rotate feature displayed)
- Over 1000 times continuum emitting
- Quartz tube design
- Socket for power pack
- Extreme High Heat Resistance
- etc

After a few hours of playing I have mixed feelings about it. Just a few "hot" impressions next.

1. The build quality is very good, on par with Nikon units.
2. The color LCD is very nice and the menu is very intuitive... I like it much better than Nikon ugly LCDs. Also it is very visible in the dark... a plus for a person with myopia like me
3. I did a unscientifically test in manual mode at full power in parallel with my SB910 and I think the two units have about the same power, not like advertised. So no extra juice in this unit if you expect this...
4. You can use it manual mode from 1/1 to 1/128... What's very nice is that you can go in 1/3 stops between so you can adjust exactly the power you need.
5. When using only with internal batteries the interval between two emissions is probably twice longer than on SB910, especially when is close to full power. That's really bad.
6. When using at 1/16... 1/32 you can have very frequent emissions... and no problems with heat at all... To have the same at a higher power level you need a battery pack...
7. Mounted through a SC28 chord works only in manual mode on my D800.... surprisingly in iTTL refuses to fire. I dunno if this is a "feature" or my copy has a problem... I need to check this... My SB910 has no problem. Curiously on D70s works as a charm with the same SC28.
8. I tested it, including CLS mode on D800, D600, D7000 and even on the ol' good D70s (at 1/500...). Works with no problem in ITTL and in manual mode.
9. A very nice feature (for me) is the sub-flash... This one can be setup independently to work between "off" and "on" going through 1/128... up to 1/1 power, only when the main light is in BL. This is a very nice feature I wish Nikon will introduce in its own line. You can "spill" this way some light in order to control shadows. I show some examples shot with D70s:

I shot handheld, no tripod, and RAWs were simply converted and re-dimensioned in ViewNX2 without any other PP:

Having said this... I think this flash unit could ve very helpful for some situations. The sub-flash and also the incredible resistance to heat are very good especially when using on camera on heavy-duty situations. I also like its display and the great visibility after dark... With a battery pack it also rulz with an umbrella or softbox...

I wonder if other people purchased this flash... curious to hear other opinions and experiences.

I have the Nissin Di-866MKII. I also have the complaint that it won't fire in TTL using any TTL cord but the one made by Nissin. I actually found this stated somewhere online and along with it was some statement about different voltages or something. It seems improbable to me that it should be able to fire with any old TTL cord but it won't. I complained to Nissin about this issue and aside from a bit longer recylce time it's my only complaint that I can think of about the di-866 MKII (which is very similar to the MG8000 but without the heat protection). I currently trigger it with either the pop-up, the Pixel King radio trigger or my Phottix Stratos trigger.

I imagine the MG8000 would be awesome when paired with the Nissin battery pack.

Thanks Brad for bringing more light in the chord issue. Yes, this is bad but... after I played today more with the flash I can definitely say that I like it very much. I have no the (expensive) Nissin power pack but with the one from Nikon (for SB900/910) the recycle performance is much better, maybe twice faster than with internal batteries. I think that this will be my default flash on camera / or bracket from now ahead.

I have the Di866II in the Canon version and I have no complaints. I have 3 Canon Off Camera Shoes Cord 2 and the Nissin will not work with one of them. Seems very much on par with the Canon 580 EX flash. I like the menu system and build quality is decent as well.